1. Field of the Invention
The present invention relates to a negative electrode material for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery that uses a negative electrode material for a non-aqueous electrolyte secondary battery.
2. Description of the Related Art
In recent years, with a remarkable development of portable electronic devices, communication devices and electric cars, from the viewpoint of economic efficiency, long life of devices and small size and light weight, a non-aqueous electrolyte secondary battery having high capacity and high energy density is in strong demand.
Accordingly, a silicon-based active material that is high in theoretical capacity as a negative electrode material is gathering attention. For example, in patent document 1, by using silicon oxide in a non-aqueous electrolyte secondary battery, for example, as a negative electrode material for a lithium ion secondary battery, an electrode having high capacity is obtained. However, there are problems such that irreversible capacity at the initial charge/discharge is large, and the cycle property does not reach a practically-usable level, that is, there is a room for improvement.
Further, in a non-aqueous electrolyte secondary battery that uses a silicon-based active material in a negative electrode material, gas is observed to evolve in a cell. Since this is an issue largely related to safety and reliability of a battery, a countermeasure is in demand.
Regarding the reason why the gas is generated within a battery, the following mechanism is assumed.
It is known that LiPF6, which is used as an electrolyte in a general lithium ion secondary battery, reacts with water according to a reaction shown in the following chemical reaction formula (a).LiPF6+H2O→LiF+2HF+POF3  (a)
In addition, it is known that SiO2 reacts with HF according to the following chemical reaction formula (b).SiO2+4HF→SiF4+2H2O  (b)
That is, it is considered that in a battery that uses a silicon-based active material in a negative electrode, according to a reaction shown by the chemical reaction formula (a) between water that is slightly present within a battery and LiPF6 that is an electrolyte, HF gas is generated, and the HF gas reacts with SiO2 that is contained in the silicon-based active material according to the chemical reaction formula (b) to generate a gas. It is further considered that, since water is generated according to a reaction shown by the chemical reaction formula (b), the two reactions are repeated inside a battery, a large amount of gas is generated. Accordingly, it is considered effective to modify a surface of particles to suppress the reaction according to the chemical reaction formula (b) to suppress gas generation inside a cell.
Regarding a silicon-based active material in which a surface of particles is modified, Patent Documents 2, 3, 4 etc. have been reported, and regarding a negative electrode that is obtained by forming a fluororesin film on a negative electrode active material layer, Patent Documents 5, 6, 7 etc. have been reported. However, according to the study by the present inventors, when a negative electrode material and a negative electrode prepared according to the similar method were subjected to a gas generation test, gas generation was found. That is, from the viewpoint of safety and reliability of a battery, a further improvement is necessary.